1. Field of the Invention
This invention relates generally to open cycle refrigerators and more particularly to cryogenic coolers which employ expander pistons with counterflow passageways.
2. Description of Related Art
Over the past several decades, compact cryogenic refrigerators have been developed to give cryogenic temperatures from about 8.degree. K. to 150.degree. K. One conventional refrigeration arrangement is the Joule-Thompson refrigerator. In a Joule-Thompson refrigerator, incoming compressed gas from a source of compressed as such as a storage bottle passes through a counterflow heat exchanger to an expansion valve. As the gas passes through the expansion valve it is cooled and liquefied. The liquid is collected in a container. The liquid draws heat through the container from a source to be cooled. The heated liquid evaporates and is channeled through the counterflow heat exchanger and ultimately dumped into the atmosphere. As the evaporated liquid passes through the heat exchanger, it picks up additional heat from the incoming compressed gas, thereby precooling the incoming gas. While the Joule-Thompson refrigerator itself is somewhat compact it must be supported by a relatively high pressure gas source. This, coupled with the fact that the Joule-Thompson cycle is irreversible and inherently inefficient, necessitates a large gas storage volume. In applications where volume and weight are critical, the Joule-Thompson refrigerator is disadvantageously large and heavy.
Another conventional refrigeration device employing the Solvay cycle includes an expander piston containing a regenerative heat exchanger. While this cycle is typically used in a closed cycle manner (where the venting gas is recompressed), it can also be used in an open cycle manner (where the venting gas is dumped into the atmosphere). This technique allows operation with a compressed gas source and lends itself for rapid cooldown applications due to high flow capability during cooldown in a similar manner as the Joule-Thompson cycle. In such an application, the piston is located within a fluid-tight enclosed chamber, and forms a cold volume at one end of the chamber. In operation, fluid passes through the regenerator to the cold chamber and back through the regenerator whereby the piston reciprocates and the open cycle Solvay refrigeration cycle is accomplished. A drawback of open cycle refrigerators which employ regenerative heat exchangers is that a portion of the gas is prressurized and vented within the regenerator chamber without being used in the expansion process. The Solvay refrigeration arrangement is therefore inherently inefficient.
Today, smaller and smaller cryogenic refrigerators are being demanded to meet size and weight requirements desired by both military and commercial users. Furthermore, as miniature refrigerators are increasingly used to cool electronic devices, high efficiency and fast cooldown rates are desired.